How to render a night scene
The problem you are most likely facing is that you're render samples are too low.
This can make the denoise feature in cycles produce blotchy artifacts.

The way to fix this is by going to Properties &gt;&gt; Render &gt;&gt; Sampling



Under "Samples" section, increase the Render value gradually until you get desirable results.

Also make sure not to overdo it because setting the value high (like 1000) can have a massive hit on your computers performance.